Nowadays, presensitized plates which utilize positively working sensitizers containing diazo compounds and phenol resins as main components, and negatively working sensitizers containing acryl series monomers or prepolymers as main component are put to practical use as lithographic offset printing plates. However, such plates all have low sensitivity, so images are reproduced in these plates through contact exposure using, as printing master, silver salt photographic films in which the images have been recorded in advance. Through advances in computer aided image processing, mass data storage and data communication techniques, on the other hand, electronic editing systems, wherein input, correction, editing, layout and page allotment of originals are consecutively performed by operating a computer and the resulting copies are taken out in real time as the output of terminal plotters installed in remote places by utilizing a high speed communication network or satellite communication, have been put to practical use in recent years. In particular, the field of news printing, in which rapidness is required, has the most crying need of the electronic editing system. In addition, in the field of keeping originals in the form of master films and reproducing printing plates therefrom as occasion arises, it is expected that originals will be stored in recording media in the form of digital data with the development of recording media having very large capacity, such as optical discs.
However, scarcely any direct reproduction system for making printing plates directly from the output of a terminal plotter has been put to practical use as yet. In the present situation, though an electronic editing system is at work, yet the output is recorded in a silver salt photographic film, and the resulting film is superposed on a presensitized plate and subjected to contact exposure, whereby the output is indirectly recorded in the presensitized plate to make a printing plate. This is because it is difficult to develop direct reproduction type printing plates having sensitivities high enough to make printing plates in a practical time using the output of a plotter as the light source (e.g., He-Ne laser, semiconductor laser).
Under these circumstances, electrophotographic photoreceptors are anticipated to be usable as photosensitive materials having such high sensitivities as to provide direct reproduction type printing plates.
As for the printing plate materials (plate precursors) utilizing electrophotography, there have so far been known zinc oxide-resin dispersion type offset printing plate materials disclosed, e.g., in JP-B-47-47610 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-48-40002, JP-B-48-18325, JP-B-51-15766, JP-B-51-25761, and so on. In using those materials as printing plates, toner images are formed on the materials by electrophotography, and then the materials are dampened with a desensitizing solution (e.g., an acidic aqueous solution containing a ferrocyanide or a ferricyanide) in order to desensitize the nonimage part. Though the thus processed offset printing plates have a printing impression capable of producing in the order of from 5,000 to 10,000 sheets, they are unsuitable for the printing in which a printing impression capable of producing copies in higher than the above-described order is required. Moreover, when designed so as to have compositions suitable for desensitization, the plate materials suffer from deterioration of their electrostatic characteristics and produce images of aggravated qualities. Furthermore, the desensitizing solutions used in making the printing plates have the disadvantage of containing harmful cyanides.
In organic photoconductor-resin coated plate materials as disclosed, e.g., in JP-B-37-17162, JP-B-38-7758, JP-B-46-39405, JP-B-52-2437, etc., electrophotographic photoreceptors of the type which comprise a grained aluminium plate having thereon a photoconductive, electrically insulating layer containing, e.g., an oxazole or oxadiazole compound bonded with a styrene-maleic anhydride copolymer are employed. After toner images are formed on those photoreceptors through electrophotography, the nonimage part is removed by dissolution in an alkaline organic solvent to make a printing plate.
As a representative of the above-described printing plate materials, the Elfasol system is put on sale from Kalle A. G. This system is inferior in resolution because a dry developer is used therein, so it cannot provide high quality images. However, dry developers are, in general, superior in resit quality to etching solutions since they are particles constituted, in general, with a coloring agent and a resin, and measuring from 10 microns to several tens of microns to form a thick toner layer on the image part.
A particle size of toner to form images, as well as known, can be rendered smaller (0.1 to 2 microns) in a liquid developer than in a dry developer, so the liquid developer can provide high quality images excellent in resolution.
However, using a liquid developer in the systems as described above causes a trouble that even the image part tends to be etched due to excellent resolution of the liquid developer upon removal of the nonimage part with an alkaline etching solution As a consequence it follows that images excellent in resolution cannot be obtained in spite of using the liquid developer. On the other hand, the attempt to use materials excellent in etching resistance is attended by the problem that such materials cannot provide high resolution characteristic of a liquid developer because of difficulty in dispersing them. In addition, some of such materials raise another question that because of their insufficient adhesiveness to photoreceptive materials, the formed toner images peel in the form of film from the surface of the photoreceptive materials during the etching procedure, and therefore the materials are of no use as the resist of the image part, notwithstanding their excellency in dispersibility and etching resistance.
For instance, a liquid developer as disclosed in JP-B-52-3306, which contains as the image forming ingredient such a copolymer as to contain vinyl acetate as a main component, is readily dissolved in an alkaline etching solution, so it cannot serve as the resist.
In the case of ethylene copolymers disclosed in JP-A-58-129438 and JP-A-61-180248 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), the toner layer comes off when soaked in an etching solution though it shows sufficient etching resistance when the surface thereof is attacked by the etching solution. In addition, the ethylene copolymers are known to be very difficult to disperse. Further, copolymers prepared from alkyl(meth)acrylates and styrene or its derivatives are disclosed in JP-A-58-139155, but they are still insufficient to function as resist. Furthermore, though terpolymers prepared from (meth)acrylates, styrenes and acidic group-containing monomers are disclosed in JP-A-58-121047 and JP-A-58-127939, they are also insufficient in resist quality. In particular, the acidic group-containing monomer component has turned out to be extremely disadvantageous when an alkaline etching solution is used. Besides the above-described copolymers, alkyd resins, phenol resins, rosin-modified resins and the like, though used as a trial, were unable to bring about desirable results. Moreover, suitable liquid developers have not yet found out among various commercially available ones.